1. Field of the Invention
The invention is directed to a synthesizer. Specifically, the invention is directed to a synthesizer utilizing variable frequency comb lines and toggling.
2. Background of the Invention
Vadim Manassewitcsh in Frequency Synthesis Theory and Design (3rd Edition, John Wiley and Sons 1987) starts Chapter 1 “A frequency synthesis is a combination of system elements that results in the generation of one or many frequencies from one or a few reference sources.” Manassewitch then goes on to explain the different types of synthesizer: coherent or incoherent; direct or indirect.
The signal source for a synthesizer can be an oscillator capable of tuning directly to approximately the desired frequency. However, most oscillators are incoherent until they are phase locked using a Phase Lock Loop (PLL) to some reference frequency. There is a large frequency difference between the desired frequency and the reference frequencies that needs to be addressed.
Some existing synthesizer designs solve this frequency difference problem using a reference signal (or, one of the reference signals) to drive a comb generator. In simple terms, the output frequencies of a comb generator are all harmonically related to the input frequency. Typically, a frequency comb spans the entire frequency band required of the synthesizer and has evenly spaced (in frequency) teeth. The teeth can be used like a ruler to measure other frequency sources with high precision.
Existing frequency synthesizers rely on fixed inputs or in some cases either a couple of harmonics or sub-harmonics of the input frequency to drive the comb generator or generators. By allowing the input frequency to vary (near) continuously over a frequency range, the ruler can be controllably stretched or compressed. These stretchable comb lines can then be compared to the oscillator output frequency by using a frequency mixer. The mixer finds the sum or the difference between the oscillator frequency and all the comb lines.
Existing synthesizers utilizing fixed (or near fixed) comb lines can use a Fine Tune Synthesizer (FTS) to tune to a small range of offset frequencies around the fixed comb lines. However, this results in tuning holes in the output frequency range due to limitations in the FTS tuning and frequency range limitations of the PLL itself. An additional disadvantage is the existence of close spurs due to mixer non-linearities and adjacent comb lines at specific Direct Digital Synthesizer (DDS) frequency settings. These spurs can result in undesirable sinusoidal phase variation over time or other degradations of the desired signal quality (spectral purity).